Low Intensity Monochromatic Red, Blue or Green Light Increases the Carbohydrate Levels and Substantially Extends the Shelf Life of Fresh-Cut Lettuce

During dark storage of fresh-cut product prepared from butterhead and iceberg lettuce the levels of carbohydrates (glucose, fructose, sucrose, starch) decrease. Carbohydrate levels were low in butterhead lettuce at the start [similar to 20 mg/g dry matter (DW)] and levels decreased by over 50% during storage at 12 degrees C; carbohydrate levels in iceberg lettuce were much higher at the start (similar to 350 mg/g DW) and decreased by over 70% during storage. When the fresh-cut lettuces were stored under a low intensity monochromatic light (red, blue, red + blue, green), either continuous or in a 12-h photoperiod, the shelf life was markedly prolonged over the dark control. The prolonged shelf life coincided with vastly increased levels of carbohydrates in the lit samples. In butterhead lettuce, the carbohydrate levels increased during storage up to 10 times compared to the initial level; in iceberg lettuce the light greatly prevented carbohydrate depletion. The shelf life showed a positive correlation with the amounts of carbohydrates in the fresh-cuts measured at day 7 or 14 of the storage. This strongly indicates that the carbohydrate level is a key factor in extending shelf life of fresh cut lettuce stored under light. The applied light intensity (5 mu mol m(-2) s(-1) PAR) is considered well below the light compensation point which indicates that photosynthetic sugar production may not be the primary process involved in sugar accumulation. The possible involvement of gluconeogenesis in sugar accumulation under low light is discussed.